Method of manufacturing an inductive device

ABSTRACT

An inductive choke coil comprises a multi-layer toroidal coil enclosing a core formed from powdered iron and a binder. The coil is first wound in linear fashion, after which a flexible tubular member is inserted therewithin and filled with the core material. The coil is then formed into a toroid, and the binder material is activated for forming a substantially continuous core.

United States Patent [151 3,659,336

Home 1 May 2, 1972 [54] METHOD OF MANUFACTURING AN [5 6] References Cited INDUCTIVE DEVICE UNITED STATES PATENTS [721 Arthur Pmlani Oreg- 1,994,534 3/1935 Robinson ..29/605 [73] Assigneez Electronic Diversified, Inc. Portland, 2,419,847 4/1947 Mittermaier ..336/233 Primary Examiner-John F. Campbell I Filed: Jan. 30, 1 7 Assistant Examiner-Carl E. Hall 21 1 pp No: 7,039 Attorney-Buckhorn, Blore, Klarquist and Sparkman [57] ABSTRACT [52] [1.5. CI ..29/6l)5, 29/602, 29/608,

An inductive choke coil comprises a multi-layer toroidal coil 264/266 264/295, 336/193 336/233 enclosing a core formed from powdered iron and a binder. [51 Int. Cl. ..H01f7/06 The coil is first wound in linear fashion, after which a flexible 58] Field of Search ..29/608, 605, 602; 336/221, tubular member is inserted therewithin and filled with the core 336/233, 198; 264/] 12, 266, 295 material. The coil is then formed into a toroid, and the binder material is activated for forming a substantially continuous core.

14 Claims, 3-Drawing Figures PATENTEDMAY 2|972 3,659,336

'- lamnnnmminimum@1 1lujnumnwamm rm1mmmmivv= ARTHUR P. HORNE INVENTOR FIG. 2

BUCKHORN, BLORE, KLARQUIST 8| SPARKMAN ATTORNEYS METHOD OF MANUFACTURING AN INDUCTIVE DEVICE BACKGROUND OF THE INVENTION Choke coils are frequently employed in electric light dimmer systems including silicon controlled rectifiers. The silicon controlled rectifiers vary the proportion of current reaching a lighting load during each half cycle of AC voltage. Without inductance in the circuit, the operation of the silicon controlled rectifiers is liable to produce radio frequency interference and lamp filament noise because of the extremely fast current rise present when the silicon controlled rectifiers turn on. Choke coils also desirably limit the current through the silicon controlled rectifiers in case of an over-current condition such as a short circuit or the like. Unfortunately, however, it is usually difficult to obtain enough inductance at the high current levels involved with a heavy lighting load because of saturation of the chokes magnetic core at heavy current levels. For example, a lighting load may normally range from 25 to 100 amperes, and under short circuit conditions a much higher current may tend to flow for a short period of time. The usual choke employed saturates at high current levels whereby no more inductance is provided than would be the case if the choke included no core of magnetic material.

A choke coil having a relatively large number of turns and a relatively low permeability core could provide adequate inductance without saturating at high current levels. However, a satisfactory device of this kind is difficult to obtain. Toroidal cores are preferred because of their relative freedom from stray fields and cross-talk between circuits, as well as a reduced tendency toward mechanical noise in adjacent parts. One method of obtaining a toroidal core in a large choke is by use of a laminated core structure. However, the laminated structure and air gaps for avoiding saturation cause these chokes to be noisy in operation. Solid ferrite cores and the like may be used, but present a problem in the winding thereof. In order to provide for the large currents involved, winding conductors must be either large or paralleled. Known toroid coil winding machines will not accommodate the heavy conductors or a number of smaller conductors employed in place thereof. Therefore, such chokes are frequently hand wound, and are therefore inclined to be expensive. Moreover, the inductance may be low because of the smaller number of turns it is practical to hand wind. Also, devices of this type are subject to the aforementioned disadvantage of saturation under short circuit current conditions and thus these devices cannot be relied upon to prevent bum-out of a silicon controlled rectifier.

SUMMARY OF THE INVENTION According to the present invention, an inductive device is provided which employs a relatively large number of turns and a continuous toroidal core of relatively low permeability. This device provides sufficient inductance to be operative in preventing radio frequency interference and lamp filament noise in silicon controlled rectifier lighting circuits and the like, and at the same time does not readily saturate. Therefore, the device is effective to limit the surge current in silicon controlled rectifiers.

An inductive device according to the present invention is manufactured by first winding in a linear fashion a coil having a relatively large number of turns, and then inserting therewithin a comminuted magnetic material with a binder therefor. The coil is then formed into a toroidal shape so the beginning of the winding substantially meets the end of the winding, while the comminuted magnetic material mixture therewithin is joined to provide a substantially continuous path. The binder is activated, as by heat, for adhering the mixture into a substantially solid and continuous body providing the toroidal core for the inductive device. The toroidal core provided may be characterized as having a distributed air gap.

It is therefore an object of the present invention to provide an improved inductive device for carrying large currents without saturating.

It is another object of the present invention to provide an improved inductive device having a relatively large number of turns and a relatively low permeability core, such device being capable of carrying large currents.

It is another object of the present invention to provide an improved inductive device employing a relatively large number of turns on a substantially solid toroidal core.

It is another object of the present invention to provide economically an improved toroidal wound coil having a ferromagnetic core and a relatively large number of turns.

It is another object of the present invention to provide an improved inductive device capable of carrying a large current without producing stray fields, which device provides a reasonable inductance without saturating at relatively high current values.

It is another object of the present invention to provide an improved toroidal high current choke which is effective at relatively high current values.

It is a further object of the present invention to provide an improved method of forming a high current inductive device with a toroidal core.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements.

DRAWINGS FIG. 1 is a side view, partially broken away in cross section, illustrating a first step in manufacturing an inductive device according to the present invention;

FIG. 2 is a side view, partially broken away, illustrating another step in manufacturing an inductive device according to the present invention; and

FIG. 3 is a plan view, partially broken away in cross section of an inductive device according to the present invention, said view also illustrating a step in forming the inductive device according to the present invention.

DETAILED DESCRIPTION Referring to the drawings illustrating an inductive device or choke 10 according to the present invention, such device is toroidal in configuration, which is here taken to mean describing a closed path. The device illustrated includes semicircular end portions 12 and 14 joined by substantially straight side portions 16 and 18. This exact configuration was suitable for the particular mounting of an inductive device according to the present invention and is not critical to the invention. The core 36' of the device is substantially continuous and comprises a mixture of comminuted magnetic material such as iron powder together with a binder therefor, which binder joins the comminuted magnetic material forming a substantially continuous and solid magnetic core. The core may be characterized as having a distributed air gap with the binder material interspersed throughout the magnetic material fon'ning such distributed air gap such that saturation for the device is unlikely to occur even at high current levels.

The winding which substantially encloses the core suitably comprises multiple helices of insulated conductors in close spaced relation defining therewithin a toroidal shaped enclosure within which core 36' is located. The helices are generally formed of fairly heavy wire, e.g. No. 13 wire, and several strands are wound together at the same time in a given layer. The strands thus wound are paralleled in electrical connection in order to increase the current carrying capacity of the device whereby the equivalent of a larger size winding wire is secured. The winding is completed in several layers comprising several hundred turns in all. Typically, the overall winding may comprise 250 to 300 turns, providing an inductance on the order of a millihenry. Enamel covered aluminum wire is preferred for greatly reducing the weight of the device.

The overall coil comprises a beginning 24 and an end 22 disposed in abutting relation with a phenolic tubular coupling member 28 extending within such beginning and end in enclosing relation to the core 36' in this region. The tubular coupling member 28 is also positioned within ends of a flexible sheath 26, suitably formed of 1/32 inch to 1/16 inch thick, flexible vinyl plastic, the use of which is hereinafter more fully described and which is also disposed in enclosing relation to core 36. The phenolic coupling member is small enough in diameter to fit inside the ends of sheath 26.

' As can be seen, this inductive device employs a relatively large number of turns on a continuous toroidal core. This device provides sufficient inductance to be operative in preventing radio frequency interference and lamp filament noise in silicon controlled rectifier lighting circuits and the like, but at the same time does not readily saturate. Therefore, the device is effective to limit the surge current in such silicon controlled rectifiers.

Turning to the method of manufacturing the above described inductive device or choke coil, such a coil is wound in a linear fashion upon a tapered arbor (not shown) employed upon a lathe or other coil winding device for winding a conventional linear winding. This winding, illustrated at in FIG. 1, is suitably wound with a number of strands, as hereinbefore indicated, forming plural helices which are paralleled to increase the current carrying capacity. Also, the helices continue in several layers. Since the arbor upon which the coil or winding 20 is wound is tapered, the coil will have a larger diameter at beginning 24 than at end 22. In the present example, the difference in diameter between the beginning and ending of the winding was approximately one-eighth of an inch.

Since coil 20 is tapered, it is readily removable from the tapered arbor, and a flexible vinyl plastic sheath 26 is then inserted therewithin. This sheath is adapted to match the interior toroidal shaped enclosure provided by the toroidal winding, e. g. near its larger end, and is suitably inserted by rolling the sheath about its longitudinal axis so that is assumes a somewhat smaller diameter, after which the sheath is extended within coil 20. The sheath 26 should be longer than the coil, for example by several inches, inasmuch as the coil will later be shaped into the configuration of a toroid whereby the sheath will be drawn up within the coil. In a particular example of an inductive device having a completed length (along the major diameter in FIG. 3) of approximately 14 inches, approximately 6 inches of such sheath extended to the right of the coil as viewed in FIG. 1. The right end of the sheath is suitably crimped over after insertion and a compressed air tube is inserted into the left end of the sheath in FIG. 1, which inflates the sheath so that it fills the linearly shaped enclosure within the coil.

The phenolic coupling member 28 is then inserted into sheath 26 at the small end 22 of the coil, and the sheath and phenolic coupling member 28 are drawn up within the coil as illustrated in FIG. 1. The diameter of tubular coupling member 28 is chosen such that it is too large to fit easily within coil 20 when sheath 26 is also within coil 20. The tubular coupling member 28 can only be wedged in the small end 22 as shown in FIG. 1, but in this manner holds the sheath 26 in position, and also positions itself in extended relation from the left end of coil 20 as illustrated in FIG. 1. A cap or plug 30 is inserted in the remaining or open end of coupling member 28.

After thus inserting plug 30, the coil 20 is disposed in an upright position on a flat surface with the plug down as illustrated in FIG. 2, and a funnel 32 is inserted within the open end of sheath 26. A thorough mixture 36 of comminuted magnetic material together with a binder therefor is poured into funnel 32, e.g. from a scoop or container 34. This mixture in the case of a particular example was comprised of ten parts by weight of powdered iron to one part by weight of a temperature curing epoxy resin in powdered form. The epoxy resin employed in a particular instance was Scotchcast 260 electrical resin manufactured by the Minnesota Mining and Manufacturing Company, St. Paul, Minnesota. A quantity of this mixture is deposited into sheath 26 within coil 20, and then the powdered mixture is tamped down with a wooden tamping rod (not shown) in order to compact such mixture as much as possible. The sheath 26 is substantially filled to the top inasmuch as the filled sheath will later be drawn within the coil.

The coil is now formed or bent into a toroidal configuration as illustrated in FIG. 3, and plug 30 is removed. It is gradually shaped until it fits within a mold box, indicated at 38, and provided with a spacer 40 inserted in a central aperture of the coil so as to maintain the desired shape adaptable for a given support structure in an electrical cabinet. The sheath 26 becomes drawn into the coil and the phenolic coupling member 28, which has an outside diameter less than the inside diameter of the larger end 24 of the coil, is inserted into such larger end. The sheath will have pulled within the coil because of the increased length of path developed by the longitudinal coil enclosure such that no interference or overlapping will take place between the beginning and end of the sheath. However, the phenolic coupling member 28 is easily insertable into the large end 24 of the coil as well as into sheath 26 at this point. The phenolic coupling member 28 is gradually worked into large end 24 of the coil and the powdered mixture is gradually compressed so that the device will fit within mold box 38.

The purpose of the sheath 26 is to hold the powder or comminuted material mixture within the enclosure provided inside the toroidal winding when the coil is formed or bent into the shape illustrated in FIG. 3. Thus, the winding at end portions 12 and 14, although originally closely spaced, tend to spread or fan at their outer perimeters whereby the powder could be lost through the coil turns preventing the formation of a continuous core. Therefore, sheath 26 is desired at least within end portions 12 and 14 to prevent loss of material. As an alternative to prevent the loss of the powdered mixture in this region, the coil may be wrapped with wide mylar tape at the end portions where fanning takes place. Preferably, the tape is wound on the first layer of coil when the coil is being wound on a tapered arbor, and then the remaining layers are wound on top of the tape. This, of course, prevents the powder from passing beyond the first winding layer.

After the device is placed in mold box 38, this box is placed in an oven where the temperature is raised to 300 F. The device is maintained at this temperature until the interior of the core rises to this temperature for at least a short period of time. As the powdered binding material reaches this temperature, it Iiquifies and cures in a short time and thoroughly binds the comminuted magnetic material such as powdered iron so that an overall continuous and substantially solid core is formed. After the device is formed in this manner, it is removed from the oven and allowed to cool. Then the device is immersed in material for impregnating and protecting at least the exterior of the device. For example, the device may be impregnated with varnish. However, a liquid epoxy resin is preferred. In a particular instance the resin employed was Stycast 3050 manufactured by Emmerson and Cumming, of Gardena, California. After activation with the catalyst, the choke coil is immersed therein for impregnating the same.

Although particular products are mentioned in this application by way of example, it is understood the invention is not limited thereto. A heat activated epoxy resin is preferred for a binder in the core mixture. However, it is apparent that the binder may comprise other materials and/or may be activated in another manner as by providing a catalyst therefor before the mixture is inserted within the coil, with the choke being completed before curing takes place. Other magnetic materials such as comminuted ferrite materials may be employed in place of iron powder, although iron powder or the like is more economical. Although an oblong construction is illustrated for the device in FIG. 3, it is apparent the toroidal device could be circular, or have another shape still describing a closed magnetic circuit and provided with a multiple turn winding thereon. I

The inductive device or choke according to the present invention is useful in light dimming equipment, not only for preventing high frequency noise and vibration of lamp filaments, but also in the protection of silicon controlled rectifiers against heavy overload currents, e.g. in the instance of a short circuit. The present device does not saturate on high overload currents, but continues to operate to provide the desired inductance. Moreover, the present choke coil may simultaneously form part of an excellent filter circuit which prevents fast overvoltage transients from reaching the light dimmer equipment. The choke coil has a relatively large number of turns and iseasily provided with nearly any desired number of turns on a relatively low permeability toroidal core with a distributed air gap.

While I have shown and described preferred embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects. 1 therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

lCLAIM: 1. The method of forming a toroidal inductive device comprising:

winding at least one length of wire about a longitudinal center line to provide a coil having a beginning and an end and a hollow interior passage therebetween, placing a preformed flexible sheath in substantial contact with turns of said coil to provide a tubular walled configuration, mixing a comminuted magnetic material with a binder therefor, inserting said mixture within the flexible sheath substantially along the length of the coil, with said mixture being restricted within such sheath, forming said coil, as provided with said sheath and with said mixture within said sheath, into a toroidal shape so that the beginning of said coil substantially meets the end of said coil, and joining said mixture to provide a substantially continuous path of magnetic material and binder, and activating said binder for forming said mixture into a substantially solid core for said coil wherein particles of said comminuted magnetic material are secured by said binder. 2. The method according to claim 1 wherein said flexible sheath is inserted in tubular form within the axial interior of said coil prior to deposition of said comminuted material within said coil inside of said sheath.

3. The method according to claim 1 including placing a tubular coupling member within' the inner diameter of and between the beginning and the end of said coil for completing said toroidal enclosure within which said comminuted material is disposed.

4. The method according to claim 2 including placing a tubular coupling member within the inner diameter of and between the beginning and the end of said coil for completing said toroidal enclosure within which said comminuted material is disposed, at least one end of said coupling member extending within an end of said flexible sheath.

5. The method according to claim 1 wherein said binder comprises a heat'activated resin in powdered form, said activating including heating said coil formed in a toroidal configuration to a temperature for activating said resin and bind forming with said comminuted material therewithin is placed in a form box prior to activation of said binder for constraining said toroidal coil into a predetermined toroidal configuration.

9. The method according to claim 7 wherein said flexible sheath is wrapped upon a portion of said winding prior to deposition of said comminuted material within the interior of said coil.

10. The method according to claim 9 wherein said winding is wound in layers and said sheath is disposed between selected layers.

11. The method according to claim 1 further including impregnating of said core with a protective material after activation of said binder.

12. The methodaccording to claim'2 .wherein a tubular coupling member is inserted inside one end of said sheath prior to forming of said core and wherein said tubular member and sheath are drawn up at least'partially within the same end of said coil,

said method further including placing a plug in the exposed end of said tubular coupling member after which said comminuted magnetic material and binder are poured into the sheath extending from the beginning of said coil.

13. The method according to claim 12 further including tamping down of said comminuted material and binder within said tubing.

14. The method of forming a toroidal inductive device comprising:

winding at least one length of wire to provide a coil having a beginning and an end and a hollow interior extending therebetween,

locating a prefonned flexible sheath of tubing within said hollow interior,

inserting a comminuted magnetic material within said sheath substantially along the length thereof to provide said mixture within said sheath entirely within the turns of said coil, forming said coil, as provided with said sheath and with said mixture within said sheath, into a toroidal shape so that the beginning of said coil substantially meets the end of said coil, and joining said mixture to provide a substantially continuous path of magnetic material and binder. 

1. The method of forming a toroidal inductive device comprising: winding at least one length of wire about a longitudinal center line to provide a coil having a beginning and an end and a hollow interior passage therebetween, placing a preformed flexible sheath in substantial contact with turns of said coil to provide a tubular walled configuration, mixing a comminuted magnetic material with a binder therefor, inserting said mixture within the flexible sheath substantially along the length of the coil, with said mixture being restricted within such sheath, forming said coil, as provided with said sheath and with said mixture within said sheath, into a toroidal shape so that the beginning of said coil substantially meets the end of said coil, and joining said mixture to provide a substantially continuous path of magnetic material and binder, and activating said binder for forming said mixture into a substantially solid core for said coil whereiN particles of said comminuted magnetic material are secured by said binder.
 2. The method according to claim 1 wherein said flexible sheath is inserted in tubular form within the axial interior of said coil prior to deposition of said comminuted material within said coil inside of said sheath.
 3. The method according to claim 1 including placing a tubular coupling member within the inner diameter of and between the beginning and the end of said coil for completing said toroidal enclosure within which said comminuted material is disposed.
 4. The method according to claim 2 including placing a tubular coupling member within the inner diameter of and between the beginning and the end of said coil for completing said toroidal enclosure within which said comminuted material is disposed, at least one end of said coupling member extending within an end of said flexible sheath.
 5. The method according to claim 1 wherein said binder comprises a heat activated resin in powdered form, said activating including heating said coil formed in a toroidal configuration to a temperature for activating said resin and binding said comminuted material into a substantially solid core.
 6. The method according to claim 1 wherein said comminuted material comprises iron powder.
 7. The method according to claim 1 wherein said coil is first wound longitudinally along a straight arbor and is then removed from said arbor prior to application of said sheath and said comminuted material therewithin.
 8. The method according to claim 1 wherein said coil after forming with said comminuted material therewithin is placed in a form box prior to activation of said binder for constraining said toroidal coil into a predetermined toroidal configuration.
 9. The method according to claim 7 wherein said flexible sheath is wrapped upon a portion of said winding prior to deposition of said comminuted material within the interior of said coil.
 10. The method according to claim 9 wherein said winding is wound in layers and said sheath is disposed between selected layers.
 11. The method according to claim 1 further including impregnating of said core with a protective material after activation of said binder.
 12. The method according to claim 2 wherein a tubular coupling member is inserted inside one end of said sheath prior to forming of said core and wherein said tubular member and sheath are drawn up at least partially within the same end of said coil, said method further including placing a plug in the exposed end of said tubular coupling member after which said comminuted magnetic material and binder are poured into the sheath extending from the beginning of said coil.
 13. The method according to claim 12 further including tamping down of said comminuted material and binder within said tubing.
 14. The method of forming a toroidal inductive device comprising: winding at least one length of wire to provide a coil having a beginning and an end and a hollow interior extending therebetween, locating a preformed flexible sheath of tubing within said hollow interior, inserting a comminuted magnetic material within said sheath substantially along the length thereof to provide said mixture within said sheath entirely within the turns of said coil, forming said coil, as provided with said sheath and with said mixture within said sheath, into a toroidal shape so that the beginning of said coil substantially meets the end of said coil, and joining said mixture to provide a substantially continuous path of magnetic material and binder. 